In the past ten years, there has been a widespread utilization of a rotating laser beam as an overhead reference plane to control the operation of earth working implements such as graders, scrapers and trenchers. One of the most common earth movers is the backhoe, which is available either as a self-contained unit or as an attachment for the rear end of a tractor, which may have a loader mounted on its front end. The popularity of such units with many small contractors is such that the units are often employed for specific excavations for which the backhoe is normally not capable of producing acceptable work. For example, if a trench is required for drainage tile with the trench bottom having a prescribed pitch relative to the horizontal, the small contractor will attempt to use his backhoe to dig such trench and this is a practical impossibility for, as is well known, the normal digging stroke of a backhoe involves an arcuate movement of the bucket throughout the stroke and to convert that arcuate movement to a linear movement parallel to a prescribed plane is a matter that is beyond the skill of the operator manipulating the various hydraulic cylinders that control the three primary pivoted elements of the backhoe.
There is, therefore, a definite need for a method and apparatus for controlling the operation of a backhoe to cause the digging edge of the bucket to move in a linear path parallel to a prescribed reference plane. While a very effective overhead reference plane may be provided by a rotating laser beam, there has not heretofore been available any concept or apparatus for automatically controlling the operation of a backhoe by such reference plane of laser energy.
In U.S. Pat. No. 3,997,071 to Teach, there is disclosed an apparatus for indicating the effective depth of the teeth of the bucket of the backhoe through the utilization of an overhead laser beam reference plane, but this apparatus in no manner provides for the automatic control of the path of movement of the bucket of the backhoe. In U.S. Pat. No. 4,129,224 to Teach there is disclosed an arrangement for controlling the path of movement of the bucket of the backhoe but this control is effected by maintaining a trigonometric relationship between the three primary angles involved in the operation of a backhoe, namely the angle between the outreach boom and the horizontal, the angle between the outreach boom and the downreach boom, and the angle between the downreach boom and the bucket which is pivotally mounted to the end thereof. While this apparatus discloses an overhead laser beam reference plane, it is utilized solely for calculating and indicating the depth of the digging teeth of the bucket, and not for actually controlling the movement of the bucket.
The apparatus disclosed in the aforementioned Teach U.S. Pat. No. 4,129,224 requires the utilization of angular transducers at each of the three primary pivot points of the backhoe, plus a micro processor for effecting the required trigonometric calculations to develop the control signals for maintaining the digging teeth of the backhoe moving along a desired plane.